In recent years, high quality of the coated paper for printing is more and more demanded as the printed product of fine visualization and colorization has been developed such that it becomes more and more important to have high quality in appearance of the final printed product such as print gloss and quality of white paper such as glossiness of white paper, smoothness, and whiteness.
It is known a method of adding satin white as a pigment to a pigment coating liquid used to produce coated paper for printing is a easy method of imparting high smoothness, white paper glossiness, degree of whiteness, and degree of opaqueness to the coated paper for printing. Here, satin white is a general name of a compound in the alumina complex system and has the chemical name of calcium trisulphoaluminate whose chemical formula is 3CaO.Al2O3.3CaSO4.31-32H2O.
This satin white is available as a white pigment having a needle-like crystal structure and a satin white product having very small particulates and homogeneous particle shapes is in particular excellent for a blade coating process of coated paper for printing among such satin white products.
As a basic reaction condition in producing the satin white, a method of mixing an aqueous aluminum sulfate Al2(SO4)3 solution with an aqueous suspension of calcium hydroxide Ca(OH)2 is known (refer to non-patent reference 1). In particular, the following points in the manufacture are noted: (1) to use high-purity reactant chemicals, (2) to add an aqueous aluminum sulfate solution to an aqueous suspension of calcium hydroxide as the order of addition, and (3) to keep the molar ratio strictly in the range from 6.0 to 7.0 wherein the molar ratio is a reaction molar ratio of calcium hydroxide to aluminum sulfate: Ca(OH)2/Al2(SO4)3.
Therefore, it is difficult to obtain desired satin white when the reaction condition is beyond such a range, which may further cause adverse effects such as production of aluminum oxide (Al2O3) and calcium sulfate (CaSO4) as by-product compounds of the satin white producing process. Thus, it is necessary to take a special care about the reaction condition in producing satin white and it is further important to control the reaction condition optimally between the calcium hydroxide and aluminum sulfate as the raw materials in order to obtain satin white having very small particulates and homogeneous particle shapes.
In the method of producing such satin white, a method of mixing the aqueous aluminum sulfate solution with the calcium hydroxide suspension and causing reaction in a batch manner is introduced (refer to patent reference 1) in addition to that explained by the above non-patent reference 1.
However, since the aqueous aluminum sulfate solution is added little by little to the calcium hydroxide suspension for a long period of time in the reaction method in the batch manner, satin white crystal particulates generated in an early stage of the reaction become larger while satin white crystal particulates generated in a late stage of the reaction cannot grow large enough and remain small. Therefore, it has been difficult to prepare homogeneous particle shapes of generated satin white particulates because differences of the reaction products are rendered in the crystal growth.
With respect to a method of producing very small and homogenous particle shapes of satin white, a preparation method for producing satin white particulates in the range from 0.1 to 2.0 μm is disclosed (refer to patent reference 2) wherein the method utilizes a continuous mixing agitator (Pipeline Homo Mixer) and comprises pouring prescribed amounts of calcium hydroxide suspension and aqueous aluminum sulfate solution into a mixer at once so as to mix and cause reaction of these materials as the mixing molar ratio (CaO/Al2O3) is kept in the range of 7.2±0.2.
On the other hand, since the reaction for producing the satin white is an acid-base reaction of the acid (aluminum sulfate) and the base (calcium hydroxide), the reaction is originally characterized by immediate and violent reaction process once both raw materials are mixed. Therefore, the satin white is formed as the reaction proceeds immediately after both raw materials are completely and homogeneously mixed at a prescribed reaction molar ratio corresponding to the point of neutralization of reaction where the reaction is completed such that both reaction raw materials are in a state ready for the reaction. In the reality, however, the aluminum sulfate as one of the reaction raw materials of the satin white is dissolved in water completely so as to be an aqueous solution and ready for the reaction whereas the calcium hydroxide as the other reaction raw material has a quite low water solubility of 0.2% and is in a state of suspension with hardly-solved calcium hydroxide such that the total amount of calcium hydroxide is not ready for the reaction.
Therefore, even if the prescribed amounts of calcium hydroxide suspension and aqueous aluminum sulfate solution corresponding to the complete reaction amounts for yielding the satin white are mixed at once as described in the patent reference 2, the reaction mixture composition temporarily lacks the calcium hydroxide necessary for the reaction as all the calcium hydroxide is not ready for the reaction with the aqueous aluminum sulfate solution due to the extremely low solubility to water, and the calcium sulfate is in excess such that the reaction condition is beyond the range for producing the satin white. Thus, the reaction does not stop at a stage for producing the satin white, but further proceeds to a stage where the reaction for yielding by-products such as aluminum oxide and calcium sulfate. Then, there is an issue that the satin white as a desired product cannot be obtained.
Therefore, in the producing method as describe in the patent reference 2, in order to control the particle shape of the satin white, a continuous mixing agitator is employed to mix the calcium hydroxide and the aluminum sulfate as the reaction raw materials at once. And the method is characterized in that the mixing molar ratio (CaO/Al2O3) of the calcium hydroxide suspension and the aqueous aluminum sulfate solution is kept in the range of 7.2±0.2, where the calcium hydroxide is in excess of the synthetic reaction, in order to avoid the calcium hydroxide deficient condition and the aluminum sulfate excess condition.
However, the reaction molar ratio (Ca(OH)2/Al2(SO4)3) is 6 at the theoretical reaction end point where the synthetic reaction of the satin white is completed whereas the reaction molar ratio (Ca(OH)2/Al2(SO4)3) is set to 7.2±0.2 such that the calcium hydroxide is in excess of the synthetic reaction as described in the patent reference 2. Thus, unreacted calcium hydroxide remains in the synthesized satin white such that the calcium ion (Ca2+) is released in the satin white composition from the unreacted calcium hydroxide, which was a factor to cause insufficient dispersion of satin white and the like in a pigment coating liquid if such satin white is combined into the pigment coating liquid for producing coated paper for printing.
Therefore, in the case where the satin white was combined into a coating liquid for coated paper for printing, it was necessary to take a good care of the insufficient dispersion of the satin white such that there was an issue that excess amount of disperser for the stability must be added to the satin white dispersed liquid and that sodium hydroxide must be added to the coating liquid including the satin white so as to make the liquid in strong alkaline state to yield a higher pH (pH=12-13) in order to prevent the calcium ion (Ca2+) from being dissolved.
Also, if the pigment coating liquid to which the satin white is added is made in such strong alkaline state, there was an issue that the coated paper for printing turned yellowish and deteriorated in the whiteness due to the strong alkaline such that the quality and marketability of the coated paper for printing were deteriorated.
In the method for producing the satin white by mixing prescribed amounts of calcium hydroxide suspension and aqueous aluminum sulfate solution at once to cause the reaction wherein the prescribed amounts corresponds to the theoretical amounts for the complete reaction yielding the satin white, as described in the patent reference 2, a method for producing the satin white by adding divided amounts of calcium hydroxide suspension and aqueous aluminum sulfate solution as the reaction raw materials in two respective stages instead in one stage is disclosed (patent reference 3). The method is a batch method wherein 30-90% of the prescribed amount of aqueous aluminum sulfate solution is added in the first stage and the rest amount of aqueous aluminum sulfate solution is added in the second stage.
However, the publication (the patent reference 3) discloses a manner that the calcium hydroxide suspension is added to the aqueous aluminum sulfate solution, which is characterized by the opposite order to that of the regular method for producing the satin white. Since the reaction molar ratio of the aluminum sulfate to the calcium hydroxide is in much excess in an early stage if such an addition order is applied, aluminum oxide and calcium sulfate as reaction by-products are formed such that it is difficult to obtain the satin white to be desired. Further, in the publication, the aqueous aluminum sulfate solution and the calcium hydroxide suspension are mixed and reacted with each other in a batch manner such that it is difficult to homogenize the particle diameters of the satin white.
Non-patent Reference 1: “Pigment for Coating and Filler for Paper” translated by Ooe, Reizaburo; Yoshimoto, Saburo; Issued on Jan. 20, 1989, pages 183-188. Published by Uni Shuppan Kabushiki Kaisha                Patent Reference 1: JP-H05-163017-A        Patent Reference 2: JP-S53-014692-A        Patent Reference 3: U.S. Pat. No. 3,391,995        